Examples of conventional star-shaped couplers of this kind are shown in FIGS. 1 and 2. The star-shaped coupler 1 shown in FIG. 1 comprises a plurality of optical fibers 2 which are bound and fused together by heating substantially at their centers to form a constricted bound portion 3. The coupler branches into numerous fibers on both sides of the bound portion 3. The optical fibers 4a, 4b, 4c, 4d on the left side as viewed in the figure are used as input fibers, whereas the optical fibers 5a, 5b, 5c, 5d on the right side are employed as output fibers. In use of the coupler 1, the input fiber 4a is connected to the output of one input-output device, and the output fiber 5a is connected to the input of the device. Similarly, the input fibers 4b, 4c, 4d and the output fibers 5b, 5c, 5d are connected to their respective input-output devices. As an example, if the input-output device connected with the input fiber 4a emits light, the light travels through the input fiber 4a, and then it is substantially uniformly dispersed throughout the whole cross section of the waist 3a of the bound portion 3. Thereafter, the light is substantially equally distributed to the output fibers 5a, 5b, 5c, 5d for application to their respective input-output devices.
In the star-shaped coupler 11 shown in FIG. 2 in exploded view, each one end of input optical fibers 12a, 12b, 12c, 12d is jointed together to form a bound portion 14. Likewise, each one end of output optical fibers 13a, 13b, 13c, 13d is jointed together to form a bound portion 15. The opposed end surfaces of the bound portions 14 and 15 are bonded together via a mixing rod 16, though they are shown in spaced apart relation to each other. In use of this coupler, the input fibers 12a-12d and the output fibers 13a-13d are connected to their respective input-output devices in the same way as the aforementioned star-shaped coupler 1. For example, the light signal propagated through the input fiber 12a is dispersed substantially uniformly throughout the cross section of the mixing rod 16. Then, the light is substantially equally distributed to the output fibers 13a, 13b, 13c, 13d before being applied to their respective input-output devices.
In these conventional star-shaped couplers 1 and 2, the signal propagated through the input fiber 4a or 12a, for example, is distributed to all the output fibers 5a-5d 13a-13d via the constricted bound portion 3 or mixing rod 16. However, since the input fiber 4a or 12a and the output fiber 5a or 13a are connected to the same input-output devices, the delivered light signal returns to the same device. This reduces the quantity of light fed to the other input-output devices, thereby increasing the loss in the branching. In particular, it is desired that the light signal traveled through the input optical fiber 4a or 12a, for example, be distributed to three output fibers 5b, 5c, 5d, or 13b, 13c, 13d, but it is distributed to four fibers. It is now assumed that the quantity of light received by the input fiber 4a or 12a is A.sub.0 and that the propagation loss is negligible. Then, if the light is distributed to three fibers, the quantity of light supplied to each output fiber is A.sub.0 /3. In the conventional devices, however, the light is distributed to four fibers and, accordingly, the quantity of light distributed to each fiber is A.sub.0 /4. In this way, the loss in branching is greater than the ideal configuration.